This invention relates to a process for forming electrodes, to an electrode assembly produced by this process, and to a liquid crystal display and touch screen display incorporating such an electrode assembly.
Liquid crystal displays comprise a liquid crystal material sandwiched between two substantially transparent electrode assemblies. Touch screen displays of either the resistive or capacitive types comprise a display screen (for example, a cathode ray tube) having superposed thereover two substantially transparent electrode assemblies. In both types of displays, each of these electrode assemblies typically comprises a substrate on which is deposited a conductive layer thin enough to be substantially transparent. (The term “substantially transparent” is used herein to mean that the electrodes transmit sufficient visible light so that the two superposed electrodes will not substantially obscure, nor substantially distort the color of, a liquid crystal display or touch screen display incorporating the two electrodes. Typically, commercial specifications require that the two superposed electrodes have a transmittance of at least 80% at 550 nm.) In liquid crystal displays the substrate is usually glass, whereas touch screen displays usually employ a synthetic resin (plastic) substrate for at least one electrode. The conductor is often formed from indium tin oxide or a similar metal oxide. The conductor is typically formed by depositing the oxide by sputtering or chemical vapor deposition at a high temperature, and then annealing, also at a high temperature. On glass substrates temperatures in excess of 300° C. may be used to deposit and anneal the conductor; on plastic substrates, lower temperatures must be used, with resultant higher electrical resistance in the conductor.
Alternatively, both liquid crystal displays and touch screen displays may make use of thin film electrodes comprising a metallic conductive layer sandwiched between two layers having high refractive indices; these two layers usually being formed from metal oxides. The metallic conductive layer is patterned so as to divide it into a plurality of electrodes, and conductors are attached to each of these electrodes to enable formation of the desired patterns in the liquid crystal material. Hitherto, it has usually been necessary to eliminate the high index layer remote from the substrate at the points where the conductors are attached to the electrodes, since otherwise this high index layer might present too great a resistance to current flow between the conductors and the electrodes.
Several types of processes are known for forming these thin film electrode assemblies. If high resistance can be tolerated, one can simply leave the top high index layer intact, and place the contacts thereon. Alternatively, the conductors can be provided with conductive spacers and applied under pressure to the high index layer so that the spacers punch through the high index layer and contact the electrodes. However, this process tends to be unsatisfactory because the electrical contact between the conductors and the electrodes via the spaces is not closely reproducible from one electrode assembly to the next, and hence the yield of the process is adversely affected. In a further type of process, one high index layer and the metallic conductive layer are deposited upon the substrate, and the metallic conductive layer is patterned to form the electrodes. A mask is then applied to cover the areas where the conductors are to be attached to the electrodes, and the second high index layer is deposited; the mask prevents this high index layer being applied to the points where the conductors are to be attached, since the presence of the “upper” high index layer at these points would cause too high a resistance between the electrodes and the conductors. This procedure is time-consuming and costly because of the two separate deposition steps. Also, the conductive layer is exposed to air before the upper high index layer is applied, and with certain types of conductive layer, this may pose a problem because the conductive layer may oxidize in air.
In yet another type of process, all three layers are deposited in a single operation, the upper high index layer and the conductive layer are patterned to form the electrodes, and then the portions of the upper high index layer, lying in areas where the conductors are to be attached to the electrodes, are removed. This type of process only requires a single deposition operation and does avoid any risk of oxidation of conductive layer by exposure to air. However, the selective removal of the necessary portions of the upper high index layer is difficult.
Moreover, many prior art processes for forming electrodes require the use of elevated temperatures of 200° C. or more, which in practice requires the use of glass substrates or expensive high temperature plastics (polymers are known which have glass transition temperatures above 225° C. and can thus withstand processing at such temperatures). There are many applications for liquid crystal displays (for example, in cellular telephones and other mobile electronic devices) where it would be advantageous to use less expensive plastic substrates having lower glass transition temperatures if thin film electrodes could be formed on such substrates.
The present invention provides a process for forming electrodes on a substrate which requires only a single deposition operation and which does not require selective removal of the top high index layer. Preferred forms of this process permit the use of relatively inexpensive plastic substrates.